1. Field of the Invention
The present invention relates to formation of an image by transferring an image on an image bearing member onto a recording medium, more particularly, an image forming apparatus provided with contact type charging means for imparting charge to a surface layer on the image bearing member prior to electrifying operation by electrifying means to uniformize electrified potential, and a method therefor, or an image forming apparatus from which image noise resulting from switching of applied state of bias voltage to this charged member has been removed, and further an image forming apparatus capable of removing foreign matter accumulated on a contact place between the charged member and the image bearing member, and preventing the quality of an image to be formed from being deteriorated.
2. Related Art
In an image forming apparatus of the electrophotographic method such as a copying machine, a printer, or a facsimile apparatus, it is arranged such that after an image on an image bearing member is transferred onto a recording medium, residual toner is cleaned, and thereafter, it is electrified by electrifying means (such as a corona electrifying unit) again for the next image formation. In this field of image forming apparatus, the demand for stability of an image has become very severe mainly due to causes such as generalization of color image forming apparatus in recent years. For this reason, it is more and more important to secure the uniformity in surface potential on an image bearing member in a state electrified by electrifying means. To this end, there have been proposed various techniques in which auxiliary electrifying means is provided separately from the major electrifying means, or if charge of inverse polarity occurs on the image bearing member by transfer, de-electrifying means for removing it is provided, and charge is imparted to the image bearing member by auxiliary electrifying means prior to the major electrifying means to enhance the uniformity in surface potential on the image bearing member. They are disclosed in, for example, Japanese Patent Laid-Open Application Nos. 6-289687 and 5-134589 and the like. Also, there has been proposed a contact electrifying device in, for example, Japanese Patent Laid-Open Application No. 4-249270, in which a filmy member (hereinafter, referred to simply as "film"), whose one side comes into contact with the image bearing member, is provided, and bias voltage is applied to this film to impart charge to the image bearing member for electrifying its surface.
In an image forming apparatus according to these related arts, however, since a contact charging member is used also for the main electrifying means or only for opposite-polarity charge generated on the image bearing member by transfer is de-electrified even if de-electrifying means is provided, the ununiformity of the surface potential on the image bearing member could not always be removed sufficiently. Particularly when the potential at which the surface of the image bearing member is electrified by the auxiliary electrifying means exceeds the electrified potential at the main electrifying means, there have been some cases where such surplus potential cannot be absorbed by the main electrifying means, but an image of the next cycle is formed while ununiform potential remains. Therefore, the ununiformity in the surface potential of the image bearing member may appear as noise in the image, resulting in deteriorated image quality.
As another problem, foreign matter (such as paper dust, toner, and photosensitive layer material) accumulates at a contact place between the image bearing member and the contact charging member by the use to change the charging performance, thus affecting the quality of an image to be formed. For this reason, it is necessary to maintain the charging performance by cleaning to remove the foreign matter with an appropriate frequency. As a conventional example of such cleaning, there is a contact electrifying device described in Japanese Patent Laid-Open Application No. 8-69152. In this contact electrifying device, the contact charging member is provided with two chargeable areas in such a manner that they are alternately used and retraction and cleaning are performed during alternation. In this contact electrifying device, however, the size of the contact charging member tends to become large because two chargeable areas must be provided, and further, moving means for making them movable is required. Accordingly, if this device is used, the image forming apparatus tends to become complicated and large, which is against downsizing which has been strongly demanded in recent years.
Also, since a filmy member is caused to come into contact with the image bearing member, an electrostatic attraction force occurs between the image bearing member and the film by the application of bias voltage to this film. Since this electrostatic attraction force is a resistance factor to driving of the image bearing member, the magnitude of torque required for driving the image bearing member differs depending on the applied state of the bias voltage to the film. For this reason, driving unevenness may occur on the image bearing member. Of course, it is possible to control so as to correct the driving torque of the image bearing member in accordance with the applied state of bias voltage, but it is still difficult to completely eliminate driving unevenness in the moment the applied state of bias voltage is switched. Accordingly, when the bias voltage is switched while an image formation process such as formation of an electrostatic latent image on the image bearing member is being performed, stripe-pattern (lengthwise direction of the image bearing member) noise resulting from driving unevenness of the image bearing member appears in the image, thus deteriorating the image quality. This noise is more noticeable in the case of an image forming apparatus which writes an electrostatic latent image by the digital exposure method. As a process in which switching of bias voltage while the process is being performed thus may cause noise, there is a transfer process of a toner image onto a recording medium such as a printing sheet from the image bearing member in addition to the formation of an electrostatic latent image.
The present invention has been achieved in order to solve the aforesaid problems of conventional image forming technique. Namely, the object is to provide an image forming apparatus capable of preventing the surface potential of the image bearing member from becoming ununiform, and obtaining a high-quality image free from noise, and a method therefor. Also, the object is to provide an image forming apparatus, in simple and compact structure, capable of maintaining the electrifying performance by removing foreign matter accumulated at a contact place between the image bearing member and the contact charging member to thereby maintain the image quality. Further, the arrangement is made such that the switching of the applied state of the bias voltage on the contact charging member for that end is prevented from causing image noise.